WDPN31 PGTW 180900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.5N 107.3E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 120 NM SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 11 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
OVER THE PAST 12 TO 18 HOURS, TROPICAL DEPRESSION (TD) 17W PASSED
OVER HAINAN ISLAND, AND TRANSITIONED FROM A MONSOON DEPRESSION WITH
AN EXTREMELY LARGE WIND FIELD AND RMW EXCEEDING 200NM, TO A
TROPICAL DEPRESSION WITH A SMALL WIND FIELD AND AN RMW AT MOST OF
50NM. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN
OBSCURED LOW LEVEL CIRCULATION (LLC) AND FLARING CONVECTION WHICH
IS DEVELOPING ACROSS THE SOUTHERN AND EASTERN QUADRANTS OF THE LLC
BUT BEING SHEARED IMMEDIATELY OFF TO THE SOUTHWEST BY THE MODERATE
NORTHEASTERLY SHEAR. CHINESE AND VIETNAMESE RADAR DATA INDICATES A
RATHER DISORGANIZED LOW-LEVEL STRUCTURE, WITH A WEAK ROTATION
POSITIONED SOUTHWEST OF BACH LONG VY ISLAND. THE LAST WIND REPORT
FROM BACH LONG VY AT 0600Z INDICATED EASTERLY WINDS OF 10 KNOTS.
HOWEVER, EARLIER SCATTEROMETER DATA AND GLOBAL MODEL FIELDS SHOW
THE HIGHEST WINDS ARE POSITIONED ALONG THE WESTERN AND SOUTHERN
FLANKS OF THE CIRCULATION. THE INITIAL POSITION IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON THE RADAR DATA NOTED ABOVE. THE INITIAL
INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE SINGLE
AGENCY (PGTW) DVORAK CURRENT INTENSITY ESTIMATE OF T1.5.
ENVIRONMENTAL CONDITIONS ARE MARGINALLY FAVORABLE, WITH VERY WARM
(31C) SSTS AND MODERATE WESTWARD OUTFLOW ALOFT OFFSET BY LOW TO
MODERATE EASTERLY SHEAR.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) OVER EASTERN CHINA. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TD 17W IS FORECAST TO TRACK NORTHWARD THROUGH
THE SHORT FORECAST PERIOD, ALONG THE SOUTHWESTERN SIDE OF THE
EXPANSIVE STR CENTERED NEAR SHANGHAI. TD 17W IS FORECAST TO
INTENSIFY SLIGHTLY OVER THE NEXT 12 HOURS, TO A PEAK OF 30-35
KNOTS, AS THE RMW CONTINUES TO CONTRACT AND THE CONVECTION
CONTINUES TO FLARE IN PROXIMITY TO THE LLCC. THE SYSTEM IS EXPECTED
TO MAKE LANDFALL ALONG THE COAST OF VIETNAM, TO THE EAST OF
HAIPHONG, IN THE NEXT 12-15 HOURS. AFTER LANDFALL THE SYSTEM WILL
CONTINUE TRACKING NORTHWARD ACROSS EASTERN VIETNAM AND INTO
SOUTHERN CHINA, WHILE STEADILY WEAKENING, AND ULTIMATELY
DISSIPATING BETWEEN TAU 24 AND TAU 36.

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN TIGHT
AGREEMENT, INDICATING A NORTHWARD TRACK, LANDFALL EAST OF HAIPHONG
AND ULTIMATELY POSITIONING OVER SOUTHERN CHINA. THE JTWC FORECAST
IS POSITIONED NEAR THE CONSENSUS MEAN WITH HIGH CONFIDENCE.
INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT, INDICATING SLIGHT
INTENSIFICATION THROUGH TAU 12, FOLLOWED BY STEADY WEAKENING AFTER
LANDFALL. THE HWRF IS THE MOST AGGRESSIVE, INDICATING A PEAK OF 35
KNOTS AT TAU 12, WHILE THE OTHER MESOSCALE MODELS PEAK AT 30 KNOTS.
THE JTWC FORECAST IS PLACED NEAR THE COAMPS-TC, HAFS-A AND GFS
MODELS THROUGH THE FORECAST PERIOD, WITH HIGH CONFIDENCE. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   INTENSITY 00-72 HR: HIGH//
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